Generally, a sol-gel process starts with forming a colloidal solution (a “sol” phase), and hydrolyzing and polymerizing the sol phase to form a solid but wet and porous “gel” phase. The gel phase can be dried monolithically in a controlled manner, but not under supercritical conditions, so that fluid is removed to leave behind a dry monolithic matrix having an open network of pores (a xerogel). The term “xerogel” as used herein is meant to refer to a gel monolith that has been dried under nonsupercritical temperature and pressure conditions. The dry gel monolith can then be calcined to form a solid glass-phase monolith with connected open pores. The dry gel monolith can be further densified, e.g., sintered, at elevated temperatures to convert the monolith into a porous or nonporous ceramic or glass, e.g., for forming oxide-based coatings or fibers for optical applications.
The pore size and distribution of pores in a xerogel can be varied by varying reaction parameters while forming the gel. For example, U.S. Pat. No. 4,851,150, U.S. Pat. No. 4,849,378, U.S. Pat. No. 5,264,197, U.S. Pat. No. 6,884,822, U.S. Pat. No. 7,001,568, U.S. Pat. No. 7,125,912, and International Patent Publication No. WO 2006/068797, each of which is hereby incorporated by reference herein in its entirety, describe a variety of methods for making sol-gel derived monoliths wherein in one or more reaction parameters is varied to control an average pore size and/or a pore size distribution.
Thus, a need exists for improved control over pore size and pore size distribution in silica sol-gel derived monoliths, e.g., to make narrower pore size distributions, and narrower pore size distribution in combination with small average pore size, e.g., 10 nm pore size or smaller.